Process for storing a monomer phase which is liquid under the conditions of storage

ABSTRACT

A process for storing a monomer phase which is liquid under the conditions of storage and comprises methacrylic monomers to an extent of ≧95% by weight and has been obtained by condensation out of a gaseous phase or by melting a crystalline phase, in a storage vessel, in which polymer of the monomer present in dissolved form in the liquid monomer phase is removed therefrom on the route from its generation into the storage vessel, and also a process for withdrawing liquid monomer phase from the storage vessel in which polymer of the monomer present in dissolved form in the monomer phase is likewise removed.

The present invention relates to a process for storing a monomer phase which is liquid under the conditions of storage and whose content of the monomer is ≧95% by weight in a storage vessel, the monomer being a monomer from the group consisting of acrolein, methacrolein, acrylic acid, methacrylic acid, esters of acrylic acid and an alcohol having from 1 to 12 carbon atoms and esters of methacrylic acid and an alcohol having from 1 to 12 carbon atoms, and the liquid monomer phase having been obtained by condensation out of a gaseous phase or by melting of a crystalline phase. The present invention further relates to a process for withdrawing liquid monomer phase from the storage vessel.

According to the above remarks, the term monomer(s) in this document includes acrolein, methacrolein, acrylic acid, methacrylic acid, esters of acrylic acid and an alcohol having from 1 to 12 carbon atoms and esters of methacrylic acid and an alcohol having from 1 to 12 carbon atoms. Useful alcohols include both monohydric alcohols (have one —OH group) and polyhydric alcohols (have more than one —OH group). These alcohols include in particular mono- and polyhydric alkanols having from 1 to 12, preferably from 1 to 8, carbon atoms.

This definition does not necessarily imply that these esters must be prepared by reaction of the corresponding alcohols with the particular acid. Instead, useful preparation processes are also other reactions, for example transesterifications or addition reactions.

Exemplary monomers include methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxymethyl methacrylate, hydroxypropyl methacrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, 2-propylheptyl acrylate and tert-butyl methacrylate.

Typically, monomers are obtained by chemical synthesis.

However, they are not obtained immediately in pure form but rather as a constituent of gaseous or liquid product mixtures from which they have to be removed. This removal is generally undertaken using thermal separating processes, for example absorption, desorption, fractional condensation, extraction, crystallization, adsorption, etc., or using combinations of different such thermal separating processes (cf., for example, DE-A 19 924 533, WO 2004/035 514, EP-A 1 388 533, EP-A 778 225, EP-A 1 041 062, EP-A 982 287, EP-A 982 288, WO 01/96 271, DE-A 10 336 386, DE-A 19 631 645, DE-A 19 501 325, EP-A 982 289, DE-A 19 838 845, WO 02/076 917, EP-A 1 695 954, EP-A 695 736, EP-A 778 225, EP-A 1 041 062, US 2004/0 242 826, EP-A 792 867, EP-A 784 046, EP-A 695 736, EP-A 112 5912, EP-A 1 097 741, WO 00/45 928, DE-A 2 246 480, DE-A 2 362 373, U.S. Pat. No. 5,637,222 and the prior art cited in these documents).

The pure product obtained is generally a liquid monomer phase whose content of the particular monomer is ≧95% by weight.

Typically, the last step on the route from the product mixture to the pure product consists of a condensation out of a gaseous phase or of the melting of a crystalline phase. For example, the condensation can be effected in a rectification column above the feed point of the mixture to be separated (in the rectification column) into the rectification column out of the vapors ascending in the rectification column, and the pure product can be withdrawn from the rectification column above the aforementioned feed. The mixture to be separated can be fed into the rectification column either in liquid form (rectification) or in gaseous form (fractional condensation).

In both cases, i.e. both when the liquid monomer phase is obtained by condensation out of a gaseous phase and when the liquid monomer phase is obtained by melting a crystalline phase (from the crystals), it is normally obtained free of solids on visual inspection (i.e. when viewed with the naked human eye). The liquid monomer phase typically comprises free-radical polymerization inhibitors (inhibitors for suppressing undesired free-radical polymerization), which are regularly metered in in the course of its generation and/or as the completion of such a generation. In the case of generation by melting a crystalline phase, the polymerization inhibitor system may also already be present in the crystalline phase.

The liquid monomer phase obtained as described is, as the desired pure product, then normally fed to a storage vessel in which it remains stored in liquid form until it is withdrawn therefrom for the purposes of use in chemical reactions or for the purposes of transport. Typically, such vessels are also referred to as a tank or else as a storage tank. The internal volume of a storage tank (which is typically at rest) occupiable by a liquid phase is typically from 100 m³ to 10 000 m³, frequently from 200 m³ to 1000 m³ and characteristically 500 m³ (cf., for example, Research Disclosure Database Number 513001, published in January 2007). For the purpose of maintaining the desired storage temperature, a storage tank generally has an apparatus with the aid of which (for example continuously) a portion of the stored liquid monomer phase is or can be withdrawn, conducted through a heat exchanger and then recycled into the storage tank.

Since the aforementioned polymerization inhibitor systems display their full action normally only in the presence of molecular oxygen (which in turn itself is an inhibitor), the liquid monomer phases are typically stored under a gas atmosphere comprising molecular oxygen (cf., for example, WO 2005/049 543 and U.S. Pat. No. 6,910,511). In other words, the internal volume of the storage tank occupiable by a fluid phase is filled only partly by the liquid monomer phase stored in the storage tank, and the remaining occupiable internal volume of the storage vessel is occupied by a gas phase comprising molecular oxygen. With the aid of mixing apparatus, it is generally ensured that the liquid monomer phase does not become depleted of the molecular oxygen dissolved therein.

For reasons of inhibition of an undesired free-radical polymerization of the liquid monomer phase stored in the storage tank, a maximum content of molecular oxygen in the gas phase of the storage vessel is desirable. However, a disadvantage of a high oxygen content of the aforementioned gas phase is that it necessarily also comprises evaporated monomer, and mixtures of monomer and molecular oxygen may be explosive (cf. WO 2004/007 405, DE-A 102 004 034 515, WO 2005/049 543, Research Disclosure Database Number 513 001, published in January 2007 and Research Disclosure Database Number 513 002, published in January 2007). Overall, a minimization of the oxygen content of the gas phase in the storage tank is therefore desired. Particularly advantageously, it is below the so-called limiting oxygen concentration (cf. WO 2004/007 405), below which explosive behavior of the gas mixture is not possible.

In the same way, though, a very low content of free-radical polymerization inhibitor in the stored liquid monomer phase is also desirable. One reason for this is that the polymerization inhibitors are comparatively costly active ingredients. Secondly, the monomers themselves and/or their later reaction products are frequently capable of reacting with the inhibitors to form intensely colored reaction products, which is generally undesired. Moreover, the polymerization inhibitors normally have an adverse effect (for example on the product quality) in the case of use of the stored liquid monomer phase in free-radical polymerization reactions.

Against the aforementioned background, it was an object of the present invention in particular to provide a process for storing a monomer phase which is liquid under the conditions of storage and whose content of the particular monomer is ≧95% by weight in a storage vessel which, with use of a reduced addition of substances which inhibit free-radical polymerization of the monomers, enables safe storage of the liquid monomer phase.

Accordingly, a process has been found for storing a monomer phase which is liquid under the conditions of storage and whose content of the monomer is ≧95% by weight in a storage vessel, the monomer being a monomer from the group consisting of acrolein, methacrolein, acrylic acid, methacrylic acid, esters of acrylic acid and an alcohol having from 1 to 12 carbon atoms (preferably from 1 to 8 and more preferably from 1 to 4 carbon atoms) and esters of methacrylic acid and an alcohol having from 1 to 12 carbon atoms (preferably from 1 to 8 and more preferably from 1 to 4 carbon atoms), and the liquid monomer phase having been obtained by condensation out of a gaseous phase or by melting of a crystalline phase, which comprises subjecting the liquid monomer phase, on the route from its generation into the storage vessel, to at least one separating operation to remove at least a portion (preferably at least 25% by weight, better at least 50% by weight, even better at least 75% by weight and at best the entirety (especially of the relative molecular weights specified below)) of polymer of the monomer present dissolved in the liquid monomer phase. The term “polymer” here shall also encompass oligomer. Primarily, this means polymer/oligomer obtained by (undesired) free-radical polymerization.

The wording “polymer present dissolved in the liquid monomer phase” in this document shall comprise both “dissolved in molecular form” and “dissolved in colloidal form”, but each only to the extent that the dissolved form is not perceptible visually (i.e. with the naked human eye) in the liquid monomer phase to be stored.

In-house investigations have shown that especially such dissolved polymer (in contrast to macroscopically visible polymer) has a markedly polymerization-promoting activity. This is true in particular in the case of relative molecular weights based on atomic hydrogen of ≧1000, or ≧2000 or ≧3000 of the dissolved polymer.

Useful separating operations for removing polymer present in dissolved form in the liquid monomer phase to be stored include especially all mechanical separating operations which are suitable for removing ultrafine solids from liquids. Separating operations particularly suitable in accordance with the invention in this regard are filtering and/or centrifuging separating operations. These include in particular microfiltration, ultrafiltration (nanofiltration) and ultracentrifugation (the bulk densities of oligomerized and/or polymerized monomer are just as sufficiently different from the bulk density of the corresponding monomer as the particular volume expansions; at 25° C. and 1 atm, the bulk density of monomeric acrylic acid is, for example, 1.05 g/cm³, and that of polymerized acrylic acid 1.54 g/cm³; the difference in the bulk density is based in particular on the increased space requirement of the electron cloud of an unsaturated double bond; the oligo- and/or polymerized monomers accumulate on the casing of the ultracentrifuge and can be peeled off continuously therefrom). It will be appreciated that it is also possible to employ chromatographic and osmotic processes as inventive separating operations.

Preferably in accordance with the invention, the (visually imperceptible) polymer present in dissolved form in the liquid monomer phase to be stored can be removed from the liquid monomer phase by filtration. The filter materials used may, for example, be filter webs, filter fabrics, fiber layers, sintered materials or bulk material layers (for example of finely divided quartz materials, kieselguhr, activated carbon, zeolites).

In principle, it is possible for the inventive purpose to use, for example, bag filters or candle filters. The inventive filtration task can be achieved both with sewn and fully welded, preferably multilayer filter bags made of different materials. Useful such materials include, for example, stainless steel, polypropylene, cellulose, polyester, metal fabric (stainless steel, for example chromium-nickel stainless steel) and also phenol resin-bound acrylic fibers. A material particularly preferred in accordance with the invention for filters to be used in accordance with the invention (both filter bags and filter candles) is polypropylene. Filter candles usable in accordance with the invention may, however, also be manufactured from activated carbon. In principle, useful filter candles suitable in accordance with the invention include wound candles, melt-blown candles and resin-bound filter candles. According to the invention, both the filter bags and the filter candles may be used both in single and multiple filter casings. Useful casing materials include, for example, polypropylene, stainless steel and carbon steel. Preference is given in accordance with the invention to multiple casings in which up to approx. 40 individual filter bags can be used and which enable flow rates of liquid monomer phase of up to 1000 m³/h. Installation aids or bag downholders improve the correct fit of the filter bags. For example, they reliably prevent “floating” through uncontrolled back pressure. Bursting of the filter bag is likewise prevented. Specific “collars” ensure a perfect and fixed fit of the bag downholder in the filter bag (no fraying or erosion of the filter bag).

Preference is given in accordance with the invention to filter materials (filter media or filter types) whose retention efficiency for particles having a particle diameter of ≧30 μm is at least 70%, preferably at least 80%, more preferably at least 90% or at least ≧95% and more.

Particular preference is given in the process according to the invention to filter materials (filter media or filter types) whose retention efficiency for particles having a particle diameter of ≧20 μm is at least 70%, preferably at least 80%, more preferably at least 90% or at least 95% and more.

Very particular preference is given in the process according to the invention to filter materials (filter media or filter types) whose retention efficiency for particles having a particle diameter of ≧10 μm is at least 70%, preferably at least 80%, more preferably at least 90% or at least 95% and more.

Even better, in the process according to the invention, filter materials (filter media or filter types) are used whose retention efficiency for particles having a particle diameter of ≧5 μm is at least 70%, preferably at least 80%, more preferably at least 90% or at least 95% and more.

At best, in the process according to the invention, filter materials (filter media or filter types) are used whose retention efficiency for particles having a particle diameter of ≧1 μm is at least 70%, preferably at least 80%, more preferably at least 90% or at least 95% and more.

In general, for the process according to the invention (and also for the withdrawal process still to be explained later in this document), however, filter materials (filter media or filter types) will be used whose retention efficiency for particles having a particle diameter of 0.5 μm is ≦90%, preferably ≦80%. For the process according to the invention, this generally ensures sufficient space-time yields.

The aforementioned percentages are based in each case on the total number of particles of the particular particle size. Moreover, the aforementioned percentages are based on an areal loading of 20 l/(m²·min), water as the carrier medium for the particles, a temperature of 20° C. and a pressure difference of <50 mbar, and test dust according to ISO 12103-1 A3 as the particle type.

The test design is based on the French standard NF 45-303 (cf. Filtration & Separation (Filtr. sep.) ISSN 0015-1882 CODEN FSEPAA Filtration and Separation, 1997, vol. 34, No. 3, pp. 217-223).

All aforementioned properties are fulfilled, for example, by ACCUGAF™ filter bags from Eaton Filtration, LLC., 900 Fairmount Avenue, Elizabeth, N.J. 07207 (USA) of filter models AGF-51, AGF-53, AGF-55, AGF-57 and AGF-59. They are manufactured from melt-blown polypropylene in welded form. They do not comprise any binders, any interface-active substances or any additives (for example tackifying resins). Such filter materials (filter media, filter types) manufactured only from polypropylene are found to be particularly suitable for the process according to the invention, since they incite the liquid monomer phase to be stored in accordance with the invention to undesired free-radical polymerization to a particularly minor degree.

The highest retention efficiencies are achieved using type AGF-51 (at least 90% for particle diameters ≧1 μm). With increasing identifying number, the retention efficiencies decrease. For the AGF-59 type, the retention efficiency for particle diameters of ≧30 μm is, however, still above 90%.

Alternatively, for the process according to the invention, it is also possible to use PROGAF™ or LOFCLEAR™ filter bags from Eaton. The LOFCLEAR filter bags are filter bags composed of multilayer filter material and the PROGAF™ filter bags are filter bags composed of particularly highly efficient filter medium with which effective particle reductions down to the submicron range can be achieved. LOFCLEAR filter bags are likewise manufactured from 100% polypropylene.

Alternatively, it is possible for a process according to the invention to use the high-performance bag filtes of the “HP” series (especially the HPC, HPB and HPA bag filters) from Pall GmbH, Philipp-Reis-Straβe 6, D-63303 Dreieich. They consist of three polypropylene webs arranged one on top of another; an outer supporting web which ensures a good mechanical strength of the bag filter, the middle active filter web and an inner protective web which functions as a preliminary filter.

Favorable bag filters are those whose pressure difference in the new state in the event of a loading with 10 (preferably with 20) m³/h·m² of water at 20° C., is ≦100 mbar. In general, the aforementioned value is, however, ≧5 mbar.

Also suitable in accordance with the invention are candle filters manufactured from polypropylene from FUHR GmbH (D-55270 Klein-Winternheim near Mainz, Am Weinkastell 14). These include in particular the acuraProgard membrane filter candles which are likewise manufactured completely from polypropylene. They have a thermal welded construction and are as a result free of adhesives and binders. Since their filter matrix has a two-layer structure, extremely long lifetimes can be achieved when they are used.

acuraPrograd candle filters are obtainable with retention rates of at least 99.9% for particle diameters of ≧0.2 μm, or ≧0.45 μm, or ≧1 μm, or ≧5 μm, or ≧10 μm or ≧20 μm.

The temperature of the monomer phase to be filtered in accordance with the invention should, especially in the case of use of filter media made from polypropylene, be ≦95° C.

The aforementioned temperature is preferably ≦80° C., more preferably ≦60° C., most preferably ≦40° C. and at best ≦30° C. or ≦25° C. In favorable cases, it may be down to 0° C.

Otherwise, the at least one separating operation of the process according to the invention will generally preferably be performed at temperatures of ≦95° C., preferably ≦80° C., advantageously ≦60° C., particularly advantageously ≦40° C. and at best ≦30° C. or ≦25° C. In favorable cases, it may be down to 0° C.

In the case of a filtration to be performed in accordance with the invention as described, differential pressures of from ≧10 mbar to 5 bar may be employed.

The aforementioned differential pressures are preferably ≧20 mbar and ≦3 bar, more preferably ≧20 mbar and ≦2 bar, or ≦1.5 bar. When higher differential pressures are required, the filters should be exchanged. It will be appreciated that useful filter candles for an inventive filtration are also those made from polytetrafluoroethylene (for example the acuraVent AVF filter candles from FUHR GmbH), made from borosilicate fibers (for example the acuraVent AFG-GG filter candles from FUHR GmbH), made from polyether sulfone (for example the acuraFine AFS filter candles from FUHR GmbH), and made from nylon (for example the acuraPrograd filter candles from FUHR GmbH).

It will be appreciated that it is also possible for an inventive filtration to use the acuraBag filter bags from FUHR GmbH, especially those which are manufactured from polypropylene. However, the ACCUFIT® and the ULTRAFIT® filter bags from FUHR GmbH are also suitable in accordance with the invention. Advantageously in accordance with the invention, they are likewise manufactured from polypropylene in a welded design.

In general, filter bags suitable in accordance with the invention are manufactured from several filter layers.

It is advantageous in accordance with the invention when the filter medium used for a process used in accordance with the invention does not expand significantly under the load with liquid monomer phase to be filtered. The aforementioned loading will generally be from 2 to 20 m³/(h·m²).

In general, the liquid monomer phase to be filtered, in the case of use of filter bags or filter candles, will be conducted from the inside outward through the filter type. In principle, in the case of use of filter candles, the liquid monomer phase can also be guided from the outside inward.

Typical pore sizes of the filter-active layer of filter materials to be used in accordance with the invention are from 0.1 to 300 μm, preferably from 1 to 200 μm or from 1 to 100 μm and more preferably from 1 to 50 μm or from 1 to 5 μm. As already mentioned, advantageously in accordance with the invention, a plurality of layers (for example 3, or 5, or 7 layers) are employed one on top of another.

The filtration may in principle be practiced as a pressure filtration or as a vacuum filtration. It is preferably performed as a pressure filtration. It will be appreciated that it can also be practiced with centrifugation.

The process according to the invention is especially suitable when the content of the monomer in the liquid monomer phase is ≧96% by weight or ≧97% by weight, or ≧98% by weight, or ≧99% by weight, or ≧99.5% by weight, or ≧99.7% by weight, or ≧99.9% by weight. The aforementioned and all statements in this document are relevant especially when the monomer is acrylic acid (especially when the liquid monomer phase is glacial acrylic acid).

When a filter type used in accordance with the invention is exhausted, it can be replaced by a fresh filter. Of course, the filter can be configured as an interchangeable filter in accordance with the aim. Washing by means of aqueous alkali metal hydroxide (e.g. potassium hydroxide and/or sodium hydroxide) and subsequent washing to neutrality with pure water allows, in accordance with the invention, exhausted filters to be regenerated. Especially filters manufactured from polypropylene can be incinerated without any problem after their exhaustion.

As active ingredients which inhibit an undesired free-radical polymerization of the monomers present in the liquid monomer phase (for example induced by temperature, light or other spontaneously induced free-radical formation), the liquid monomer phase to be stored in accordance with the invention may comprise any polymerization inhibitors known for this purpose in the prior art in dissolved form. Polymerization inhibitors used with preference are p-methoxyphenol (MEHQ), phenothiazine, hydroquinones, phenol (e.g. 2,4-dimethyl-6,6-butylphenol), quinones, butylpyrocatechol, diphenylamine, p-phenylenediamines, nitroxyl radicals (e.g. 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-OH-TEMPO)) and/or nitroso compounds, for example nitrophenols (and also all other polymerization inhibitors mentioned in WO 00/64947).

The inventive procedure enables not only the inhibitor content of the liquid monomer phase for safe storage but likewise the oxygen content in the gas phase present above the liquid phase in the storage vessel to be reduced. Of course, the required amount of the inhibitor used also depends upon the type of inhibitor, upon the storage temperature, upon the type of monomer and upon the specific purity of the liquid monomer phase to be stored. Based on the weight of monomer present in the liquid monomer phase to be stored in accordance with the invention, the inhibitor content is generally ≦220 ppm by weight.

When liquid monomer phase stored in the process according to the invention is withdrawn from a storage vessel, this liquid monomer phase which is to be withdrawn or has been withdrawn, before its further use for a chemical reaction or for the purpose of its transport (of transport with increased safety), will, advantageously in accordance with the invention, likewise be subjected to at least one separating operation to remove polymer of the monomer which is present in dissolved form (in molecular and/or colloidal form; invisible) in the liquid monomer phase which is to be withdrawn or has been withdrawn. In other words, the present application additionally comprises a process in which the process for inventive storage is followed by a process for withdrawing liquid monomer phase from the storage vessel and the liquid monomer phase to be withdrawn, when it is withdrawn and/or after it has been withdrawn and before its further use for a chemical reaction or for the purpose of its transport by sea by ship, or by truck or a rail vehicle (i.e. by road or by rail) (for example in a tank of a tanker vehicle, for example of a tanker ship or of a tank truck (the tank material used is frequently stainless steel); the capacity of the tank of the latter (the internal volume occupiable by a liquid medium) is generally at least 5 m³, frequently at least 10 m³ and in many cases from 15 to 40 m³ or from 20 to 30 m³; in some cases, its interior is divided into at least 2, generally at least 3 or at least 4, chambers separated completely from one another, each of which is filled independently with liquid monomer phase; the fill volume with liquid monomer phase is typically from approx. 80 to 90% by volume of the internal volume occupiable in each case by a fluid phase; the capacity of the tank of a tanker ship normally far exceeds that of the tank of a tanker truck), is subjected to at least one separating operation to remove at least a portion (preferably at least 25% by weight, better at least 50% by weight, even better at least 75% by weight and at best the entirety) of polymer (oligomer) (especially of the relative molecular weights specified in this document) of the monomer present dissolved therein. The undesired formation of such especially free-radical polymer cannot entirely be suppressed even in the case of use of an inventive storage (generally, the storage is effected at temperatures of ≦50° C., preferably ≦40 or ≦30° C., but above the melting point of the liquid monomer phase; typical storage temperatures are from 17 to 25° C.). Detailed investigations have led to the result that such polymer present in dissolved form in liquid monomer phase stored in accordance with the invention, especially in the case of use of liquid monomer phase stored in accordance with the invention for free-radical polymerizations, has an adverse effect both on the course of the free-radical polymerization and on the quality of the resulting polymer, even when the amounts present are only small. The present invention therefore also comprises a process for, for example, free-radical polymerization, which comprises polymerizing (for example with free-radical initiation) liquid monomer phase which has been stored in a storage vessel or withdrawn from the storage vessel or transported as described above, or a mixture thereof with at least monounsaturated (preferably ethylenically unsaturated) compounds other than the liquid monomer phase withdrawn from the storage vessel. Useful separating operations include all of those which have already been detailed and recommended for the process according to the invention for storing a liquid monomer phase. All remarks made there apply here correspondingly.

It will be appreciated that the filter apparatus may also be installed directly into the withdrawal line or into the vessel wall at the withdrawal point.

In other words, preferably in accordance with the invention, liquid monomer phase which is to be withdrawn or has been withdrawn from the storage vessel is also subjected to a mechanical separating operation which is suitable for removing ultrafine solids from liquids. Particularly suitable separating operations in this context here too are filtering and/or centrifuging separating operations. In particular, useful filtration methods are all of those recommended for the inventive storage process, which is why the statements regarding filtration made individually in this document with regard to the storage process according to the invention apply and are valid identically for a withdrawal filtration. It will be appreciated that the filtration apparatus may also be installed directly into the withdrawal line or into the vessel wall at the withdrawal point.

Particular preference is given to using filter materials (filter media, filter types) whose retention efficiency for particles having a particle diameter of ≧10 μm is at least 90%, preferably at least 95%.

Very particular preference is given to using filter materials (filter media, filter types) whose retention efficiency for particles having a particle diameter of ≧5 μm is at least 90%, preferably at least 95%.

However, it will be appreciated that effective filter materials (filter media or filter types) are also even those whose retention efficiency for particles having a particle diameter of ≧30 μm (or ≧20 μm, or ≧10 μm, or ≧5 μm, or ≧1 μm) is at least 70%, preferably at least 80%, more preferably at least 90% or at least 95% and more.

In particular, the ACCUGAF filter bags from Eaton of the AGF 51, AGF 53 and AGF 55 model type may be used at this point. It will be appreciated that useful filters at this point are also the candle filters from FUHR GmbH. Examples include the acuraPrograd candle filters from FUHR with retention rates of at least 99.9% for particle diameters of ≧5 μm.

Appropriately, the procedure in the withdrawal of liquid monomer phase stored in accordance with the invention from the storage tank will be to withdraw the liquid monomer phase to be withdrawn from the storage tank initially as such into a test tank (of course, the filtration recommended can also be effected actually in the course of the withdrawal (for example by means of a filter disposed in the withdrawal line or at the withdrawal point in the storage vessel)).

From the test tank, the liquid monomer phase withdrawn from the storage tank is then pumped out once or more than once in succession and, for example, recycled back into the test tank each time through a filter system as described above. When the amount retained in the filter system (for example acuraPrograd candle filter with retention rates of 99.9% for particle diameters of ≧5 μm in a multiple casing) increases insignificantly, if at all, the separating operation is terminated and the monomer phase which has been withdrawn beforehand from the storage tank and is present in the test tank is sent to its further use (for example its transport with increased safety to the user, or a free-radical polymerization or another chemical reaction). The above-described separating operation is preferably also effected at very low temperature (generally ≦95° C., better ≦80° C., preferably ≦60° C., advantageously ≦40° C., more preferably ≦30° C. or ≦25° C. and in favorable cases down to 0° C.).

The present application therefore also comprises a process for transporting a liquid monomer phase which has been stored in a storage vessel in a process according to the invention and has been withdrawn from the storage vessel in an inventive withdrawal.

Otherwise, the liquid monomer phase can be stored as described in WO 2005/049543, U.S. Pat. No. 6,910,511, Research Disclosure Database Number 513002, published in January 2007 and especially Research Disclosure Database Number 513001, published in January 2007.

When the condensation to obtain the liquid monomer phase to be stored in accordance with the invention is effected in a rectification column above the feedpoint of the mixture to be separated (in the rectification column) into the rectification column from the vapors ascending within the rectification column, and the liquid monomer phase which is thus obtained by condensation and is to be stored in accordance with the invention is then withdrawn from the rectification column above the aforementioned feed, the condensation can be brought about within the rectification column, for example, by direct cooling and/or indirect cooling. Indirect cooling can be brought about at the top of the column, for example, by conducting the ascending vapor which arrives at the top of the column through an indirect heat exchanger and converting the constituents which are condensable under the conditions of the indirect heat exchange to the liquid phase. A portion of the liquid phase thus obtained is then sent to the inventive storage and the other portion recycled as reflux liquid to the top of the rectification column. This reflux liquid then causes, in the rectification column, actually a direct cooling of the vapor ascending in the rectification column. Constituents which are not condensable in the indirect heat exchanger are conducted out of the rectification column and generally sent to their disposal. The polymerization inhibitor is added directly into the condensate. When the pure product is withdrawn below the top of the column, the phase condensed at the top is normally recycled substantially as reflux liquid into the rectification column. Typically, the reflux liquid comprises added polymerization inhibitor. The pure product withdrawn from the rectification column is thus directly likewise polymerization-inhibited. Of course, the condensation at the top of the column can also be undertaken exclusively by direct cooling. For this purpose, top condensate, once it has been obtained, is admixed with inhibitor, cooled and sprayed at least partly into the top space of the rectification column for the purpose of direct cooling. When the pure product is withdrawn below the top of the column (but above the feedpoint of the mixture to be separated in the rectification column), the cooled total amount of condensate is normally resprayed into the top space. The part of the rectification column comprising separating internals and the condensation space at the top of the column are typically separated from one another by a chimney tray. The reflux liquid is fed to the separating part. The part of the rectification column below the feedpoint into the rectification column is typically referred to as the stripping section, and the part of the rectification column above the feedpoint is typically referred to as the rectifying section of the rectification column.

When the mixture to be separated in the rectification column is fed into the rectification column in liquid form, the separation is a rectification; when the feed is effected in vaporous (or gaseous) form, the separation is a fractional condensation. As separating internals, the rectification column may comprise, for example, trays, structured packings and/or random packings. Exemplary embodiments of a fractional condensation of acrylic acid from the product gas mixture of a heterogeneously catalyzed partial gas phase oxidation of propylene and/or propane are disclosed, for example, by the documents DE-A 199 24 533, DE-A 199 24 532, WO 01/77056, DE-A 101 56 016, DE-A 102 43 625, DE-A 102 23 058, DE-A 102 35 847, WO 2004/035514, WO 00/53560 and DE-A 103 32 758.

When the monomer phase to be stored in accordance with the invention is obtained by melting a crystalline phase, this may be the result of a one-stage or of a multistage crystallizative purification of a liquid mixture comprising the relevant monomer (cf., for example, EP-A 616 998). As a result of the action of cold conditions on the liquid mixture, the monomer crystallizes out of the liquid mixture as a crystalline phase.

Frequently, the crystalline phase to be melted is the result of a one-stage crystallization.

It will be appreciated that the phase to be melted may also be the result of a fractional crystallization.

For example, the action of cold conditions on the liquid mixture comprising the monomer may be performed as a layer crystallization (cf. DE-A 26 06 364, EP-A 616 998, EP-A 648 520 and EP-A 776 875). In this method, the crystals are frozen out in the form of continuous, firmly adhering layers on cooled surfaces. The crystals deposited are separated from the remaining residual melt (the mother liquor) through simple flowing-off of the residual melt. In principle, a distinction is drawn between “static” and “dynamic” layer crystallization processes. A characteristic feature of dynamic layer crystallization from liquid mixtures is forced convection of the liquid mixture. This can be effected, for example, by pumped circulation of the liquid mixture through cooled tubes with full flow-through, by applying the liquid mixture as a trickle film to cooled walls (for example according to EP-A 616 998, for example in cooled downpipes) or by introducing inert gas into the liquid mixture or by pulsation.

In the static processes, the liquid mixture is at rest (for example in tube bundles or plate heat exchangers) and is deposited in layer form on the secondary side of the heat exchanger by slowly lowering the temperature. Thereafter, the residual melt (mother liquor) is discharged, more highly contaminated fractions are sweated off from the crystal layer by slowly increasing the temperature and then the pure product is melted off (cf. WO 01/77056).

Generally, the liquid mixture comprising the monomer (normally a solution), from which the monomer is removed by crystallization, comprises added polymerization inhibitor. In the crystallizative deposition of the monomer, it normally crystallizes out essentially free from the polymerization inhibitor. The deposited monomer crystal layer is therefore frequently not melted by heating the secondary side but rather by contacting the monomer crystal layer with a warmed melt of deposited monomer crystal layer which has been melted beforehand, said melt comprising added polymerization inhibitor.

Alternatively, the monomer crystals can also be formed from the liquid mixture comprising the monomer as a suspension crystallization (for example according to the teaching of WO 01/77056, of WO 02/055469 and of WO 03/078378).

In general, a crystal suspension comprising suspended acrylic acid crystals is obtained by cooling the liquid mixture (the liquid solution) which comprises, for example, acrylic acid as a monomer, the acrylic acid crystals having a lower impurity content and the remaining residual melt (mother liquor) a higher impurity content than the liquid starting mixture. The acrylic acid crystals may grow immediately within the suspension and/or be deposited as a layer on a cooled wall, from which they are constantly scraped off and resuspended in the residual melt.

All suspension crystallizers and suspension crystallization processes detailed in WO 01/77056, WO 02/055469 and in WO 03/078378 may be employed. An acrylic acid crystal suspension thus obtained may, for example, have a solids content of from 10 to 50% by weight, frequently from 20 to 40 or 30% by weight.

For the separation of suspension crystals and remaining mother liquor, for example, all separation processes mentioned in the aforementioned WO publications are useful (for example mechanical separating operations such as centrifugation). Preference is given to effecting the separation in a wash column. Advantageously, the wash column is a wash column with forced transport of the deposited, for example, acrylic acid crystals. The wash liquid used is advantageously the melt of, for example, acrylic acid crystals which have been purified (removed) beforehand in the wash column. The wash is normally effected in countercurrent.

All of the aforementioned is true in particular when the wash column is a wash column with forced transport of the, for example, acrylic acid crystals, in particular when it is a hydraulic or a mechanical wash column according to WO 01/77056 and it is operated as detailed there.

A liquid acrylic acid phase to be stored in accordance with the invention can thus, for example, be obtained as follows. Heterogeneously catalyzed one-stage or two-stage partial oxidation of a C₃ precursor of acrylic acid (e.g. propylene, propane or acrolein) generates a product gas mixture comprising acrylic acid. It is conducted into a separating column with separating internals and fractionally condensed ascending therein. Above the feedpoint but below the top of the column, a liquid crude acrylic acid which has ≧95% by weight of acrylic acid and comprises polymerization inhibitor added via the reflux into the separating column is withdrawn from the separating column. Suspension crystallization generates glacial acrylic acid suspension crystals from the crude acrylic acid. This is removed in a wash column (preferably in a hydraulic wash column) from remaining mother liquor using pure crystal melt comprising added polymerization inhibitor as a wash liquid. Contacting of removed pure crystals with the melt of pure crystals which have been removed beforehand which comprises added polymerization inhibitor melts the pure crystals removed and thus generates a liquid acrylic acid phase to be stored in accordance with the invention (purity generally ≧99.5% by weight).

It will be appreciated that the processes for storage and for withdrawal detailed in this document can also be applied to liquid monomer phases which have been obtained in different ways and whose content of the monomer is <95% by weight. The process according to the invention can also be performed in such a way that the liquid monomer phase to be stored (if appropriate initially as such) is conducted into the storage vessel and then liquid monomer phase continues to be withdrawn (at best continuously) from the storage tank in the course of storage and is then recycled back into the storage tank, and by this route of withdrawal and recycling is subjected to at least one separating operation to remove at least a portion of polymer of the monomer which is present in dissolved form in the liquid monomer phase. In this case, the inventive route of the liquid monomer phase leads from its generation into the storage vessel first in a simple manner through a loop in which it is now conducted out of the storage vessel once again. It will be appreciated that all inventive variants (or parts of these variants) presented in this document may also be employed in combination.

EXAMPLE AND COMPARATIVE EXAMPLE a) Comparative Example

Fractional condensation of the product gas mixture of a heterogeneously catalyzed partial propylene oxidation generated, as described in example 1 of WO 2004/035514, acrylic acid which comprised 96.9% by weight of acrylic acid and had been polymerization-inhibited with 0.018% by weight of MEHQ, 0.012% by weight of phenothiazine and 0.0004% by weight of molecular oxygen. It was withdrawn from the second collecting tray of the condensation column above the feed of the product gas mixture into the condensation column with a temperature of 100.6° C. and visually free of solids. 0.5 ml of this acrylic acid was cooled to 25° C. and transferred into a 1.8 ml glass ampoule under an air atmosphere. Subsequently, the ampoule was stored at 120° C. in a forced-air drying cabinet while being rotated, in order to ensure complete mixing. The time t up to complete polymerization of the sample was then recorded. t was 15 h 17 minutes.

b) Example

The procedure of the comparative example was repeated. After its withdrawal from the separating column, the acrylic acid was cooled to 30° C. and conducted through an ACCUGAF AGF-51 filter bag disposed in a multiple casing. After the filtration had ended, the inner walls of the filter bag had a tacky deposit of polyacrylic acid. 0.5 ml of the filtered acrylic acid was transferred into a 1.8 ml glass ampoule at 25° C. and under an air atmosphere. Subsequently, the ampoule was stored at 120° C. in a forced-air drying cabinet while being rotated, in order to ensure complete mixing. The time t until complete polymerization of the sample was then recorded. t was 20 h 26 minutes.

As the two experiments show, the removal of polymer present in dissolved form (poly-acrylic acid present in dissolved form) reduces the tendency of acrylic acid to undesired free-radical polymerization.

U.S. Provisional Patent Application No. 60/896559, filed Mar. 23, 2007, is incorporated into the present patent application by literature reference.

With regard to the abovementioned teachings, numerous changes and deviations from the present invention are possible. It can therefore be assumed that the invention, within the scope of the appended claims, can be performed differently than the way described specifically herein. 

1. A process for storing a monomer phase which is liquid under the conditions of storage and whose content of the monomer is ≧95% by weight in a storage vessel, the monomer being a monomer from the group consisting of acrolein, methacrolein, acrylic acid, methacrylic acid, esters of acrylic acid and an alcohol having from 1 to 12 carbon atoms and esters of methacrylic acid and an alcohol having from 1 to 12 carbon atoms, and the liquid monomer phase having been obtained by condensation out of a gaseous phase or by melting of a crystalline phase, which comprises subjecting the liquid monomer phase, on the route from its generation into the storage vessel, to at least one separating operation to remove at least a portion of polymer of the monomer present dissolved in the liquid monomer phase.
 2. The process according to claim 1, wherein the internal volume of the storage vessel occupiable by a fluid phase is from 100 m³ to 10 000 m³.
 3. The process according to claim 1, wherein the storage vessel has a device with the aid of which a portion of the stored liquid monomer phase can be withdrawn, conducted through a heat exchanger and then recycled into the storage tank.
 4. The process according to claim 1, wherein the liquid monomer phase in the storage vessel is stored under an atmosphere comprising molecular oxygen.
 5. The process according to claim 1, wherein the liquid monomer phase comprises dissolved p-methoxyphenol and/or phenothiazine.
 6. The process according to claim 1, wherein the at least one separating operation is filtration and/or centrifugation.
 7. The process according to claim 1, wherein the at least one separating operation is a filtration in which the filter medium used has been manufactured from polypropylene.
 8. The process according to claim 1, wherein the at least one separating operation is a filtration in which the filter medium has been manufactured from stainless steel.
 9. The process according to claim 1, wherein the at least one separating operation is a filtration in which the degree of retention efficiency of the filter medium used for particles having a particle diameter of ≧30 μm is at least 90%.
 10. The process according to claim 1, wherein the at least one separating operation is a filtration in which the degree of retention efficiency of the filter medium used for particles having a particle diameter of ≧10 μm is at least 90%.
 11. The process according to claim 1, wherein the at least one separating operation is performed at a temperature of ≧95° C.
 12. The process according to claim 1, which is followed by a process for withdrawing liquid monomer phase from the storage vessel and the liquid monomer phase to be withdrawn, when it is withdrawn and/or after it has been withdrawn and before its further use for a chemical reaction or for the purpose of its transport, is subjected to at least one separating operation to remove polymer of the monomer which is present dissolved therein.
 13. The process according to claim 12, wherein the at least one separating operation is a filtration.
 14. The process according to claim 13, wherein the degree of retention efficiency of the filter medium used for particles having a particle diameter of ≧10μm is at least 90%.
 15. The process according to claim 1, wherein the liquid monomer phase is glacial acrylic acid whose acrylic acid content is ≧99% by weight.
 16. A process for free-radical polymerization, which comprises polymerizing liquid monomer phase withdrawn from the storage vessel by a process according to claim 12 with at least monounsaturated compounds other than the liquid monomer phase withdrawn from the storage vessel.
 17. A process comprising transporting a liquid monomer phase, wherein the liquid monomer phase is a liquid monomer phase withdrawn from the storage vessel by a process according to claim
 12. 18. A process for free-radical polymerization, which comprises polymerizing liquid monomer phase transported by a process according to claim 17 or a mixture thereof with at least monounsaturated compounds other than the transported monomer phase.
 19. A process for free-radical polymerization, which comprises polymerizing liquid monomer phase withdrawn from the storage vessel by a process according to claim 13 with at least monounsaturated compounds other than the liquid monomer phase withdrawn from the storage vessel. 